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Hollow Glass Microspheres in Repair Compounds
9 Hollow Glass Microspheres in Repair Compounds Baris Yalcin
Hollow glass microspheres (HGMs) have been a key component of repair compounds for auto, wall, plaster, etc., since the 1960s. In this chapter, we will briefly discuss the use and benefits of HGMs in these application areas.
Auto Repair Compounds An auto repair compound is used to repair and patch damaged or dented sheet metals. Metal surfaces of automobile bodies (and household appliances) having dents, cracks, or holes are typically repaired with inorganic filler containing thermosetting unsaturated polyester patching compound in order to eliminate these imperfections (Figure 9.1). A satisfactory elimination of such imperfections involves subjecting the dents filled with the hardened patching material to a finishing operation in order to obtain a smooth, uniform surface which blends with the metal surface being repaired so as to be indistinguishable from the rest of the metal surface after a coat of paint has been applied to the surface. Since this subsequent finishing operation is costly and time consuming, minimizing the time required for the finishing operation is of great importance and HGMs play
Figure 9.1 Auto repair compound being applied to a sheet metal of automobile bodies. Hollow Glass Microspheres for Plastics, Elastomers, and Adhesives Compounds http://dx.doi.org/10.1016/B978-1-4557-7443-2.00009-8 Copyright © 2015 Elsevier Inc. All rights reserved.
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an important role in reducing the time required for finishing operation. In addition, HGMs render the patching material readily workable, that is, ability to file and sand, with good featherability, that is, ability to blend in with adjacent metal surfaces. Unsaturated polyesters are the typical binder ingredient of an auto repair compound. The unsaturated polyesters used in the patching compounds are usually dissolved in styrene monomer, usually 10e30 part styrene to 90e70 parts by weight of the unsaturated polyester. We have discussed the use of unsaturated polyesters in fiber reinforced sheet molding compound (SMC) applications in a prior chapter. As described in Chapter X, the unsaturated polyester itself is usually formed by the esterification of glycols with dicarboxylic acids or acid anhydrides. One obvious difference in a repair compound versus structural articles made by SMC (or the like) is the absence of glass fibers in a repair compound formula. Typical ingredients of an auto repair compound comprises unsaturated polyester resin fillers (talc, HGMs, and sometimes calcium carbonate) thixotrope and flow agent (e.g., cellulose acetate butyrate (CAB)) hardenerdadded just before using The mixture of thermosetting polyester resin binder/flow additives and inorganic fillers has the consistency of a paste. The filled polyester patching paste and the hardener (cross-linking catalyst such as benzoyl peroxide, cumyl peroxide, and methyl ethyl ketone peroxide) components are packaged in separate containers and blended together (0.5e5 wt%) immediately prior to application on a dented surface to accelerate hardening of the patching compound. At the time of application, the filled thermosetting polyester patching compound is partially gelled and thixotropic and can be formed and shaped to the configuration of the dent, and upon cure, the patching compound becomes a hardened thermoset mass of resin which is confined within the dent. Table 9.1 shows the ingredients of a metal patching compound (0.77 g/cc and 220,000 cps peak viscosity) where the fillers were incorporated in the polyester resin in major amounts, for example, the total amount of filler particles incorporated in the polyester resin constitutes one-half to twothirds the total volume of the patching composition [1]. This compound was prepared by first mixing uPS and cellulose acetate butyrate (CAB) in a low speed spiral mixer at 80 C until a clear solution is obtained and cooled to 20 C. TiO2 and the fillers (talc and HGMs) were added subsequently.
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Table 9.1 Metal patching compound (0.77 g/cc and 220,000 cps peak viscosity) US Pat. No. 3,873,475 [1] Parts Polyester:styrene solution (70:30)w/accelerator dimethyl aniline
58.7
Cellulose acetate butyrate (CAB)
1.8
TiO2 rutile
1.0
Talc (10 mm)
23.0
HGM (0.2 g/cc)
15.5
HGM, hollow glass microsphere.
After mixing to produce a uniform blend, a vacuum of 635 mm Hg was applied and mixing continued until a smooth, viscous composition, homogeneous in appearance and free of air bubbles formed. The final compound density was 0.77 g/cc and it had a viscosity of 220,000 cps. In another early reference [2] (US 4,053,448), it was shown that the proper selection of high density fillers (silica to talc ratio) was a critical feature of the formulation to provide superior workability properties. Table 9.2 shows the ingredients of the auto body repair compound discussed in this reference. The effect of varying amounts of amorphous silica/talc filler admixture on the performance of auto repair compound is shown in Table 9.3. It shows that the silica/talc ratio of 1:1 to 1:2 gives the best results as far as adhesion, fileability, and sandability. Adhesion was a measure of firm bonding of a sufficiently hardened compound to the metal surface 7 min after catalyst Table 9.2 Auto repair compound comprising silica/talc high density filler admixture and HGMs (US 4,053,448) [2] Parts Polyester:styrene solution (70:30)
42
TiO2 rutile
1.0
HGM (0.23 g/cc, 500 psi) low density filler
7
Silica/Talc high density filler admixture
50
Benzoyl peroxide
4
HGM, hollow glass microsphere.
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Table 9.3 Performance of auto repair compound with varying silica/talc filler admixture Filler Admixture
Gel
Elapsed
Silica Talc Time Adhesion
Elapsed
Fileability
Time
Sandability
Time
Paintability
Min
Rating
Min
Rating
%
Min
Rating
Rating
1
33
17
2.5
Good
Good
7
Good
12
Excellent
2
25
25
2.5
Excellent
Excellent
7
Excellent
12
Excellent
3
17
33
4.5
Excellent
Excellent
10
Excellent
14
Good
4
8
42
5.0
Good
Good
12
Good
20
Poor
5
42
8
2.5
Poor
Poor
7
Poor
12
Excellent
6
50
0
2.5
Poor
Poor
14
Poor
20
Excellent
8
0
50
5.5
Poor
Poor
14
Poor
20
Poor
9
46
4
2.5
Poor
Poor
7
Poor
12
Excellent
10
4
46
5.5
Poor
Poor
14
Poor
20
Poor
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%
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Table 9.4 Ingredients of an auto patching compound useful for repairing galvanized steel Ingredients
Wt%
uPS resin
33
Styrene monomer
15
Toluene sulfonamideeformaldehyde resin
3
Talc
44
HGM
5
HGM, hollow glass microsphere.
addition. Fileability was a measure of the ease of hand filing the hardened patching composition, that is, if the hardened patching composition was soft and pliable enough to be hand filed, after 7e14 min. Sandability was a measure of whether or not the patching composition applied to the dent can be sanded to trade specifications after 12e20 min have elapsed from the time of catalyst addition to the patching composition. Paintability was rated based on appearance being free of pinholes and blisters. In reference US 4,980,414 [3], it was shown that a uPS-based patching compound that includes aryl sulfonamide-aldehyde resin could be used for repairing metal surfaces with galvanized steel (Table 9.4). When the metal requiring repair is galvanized steel, it is difficult to ensure adhesion of the body filler to the galvanized steel surfaces. Generally, the galvanized steel surfaces are pretreated in order to provide the desirable adhesion of the body filler to the galvanized steel. The pretreatment generally involves thoroughly grinding the galvanized surface to remove the zinc coating, sanding of the surface, acid etching of the surface to remove any remaining galvanized coating, neutralization of the acid, followed by cleaning and drying of the surface. In many instances, this treated surface is coated with an epoxy/urethane primer to ensure corrosion protection (since the galvanized surface has been removed) followed by light sanding of the primer. The body filler can then be applied to the pretreated galvanized steel surfaces, and the adhesion of the body filler to the surface is generally acceptable. However, this process, which is required to prepare galvanized steel for body fillers is both time consuming and costly. Also, the corrosion protection is lost by removal of the galvanized surface provided by the primer which is often not the equivalent of the corrosion protection afforded by the original galvanized surface.
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Figure 9.2 Heavyweight versus lightweight spackle in plasterboard.
Wall Repair (Spackle Compounds) Interior walls of buildings are often constructed using gypsum wallboard panels (sometimes referred to as drywall, plasterboard). Where cavities, recesses, holes, etc., may be present (due to imperfections or damage) it is common to use wall repair compound (often referred to as “spackling”) to fill such cavities (Figure 9.2). Conventional wall repair compounds often include 1. polymeric resin binder(s) (e.g., polyvinyl alcohol (PVA) emulsion in water) 2. thickeners 3. water 4. inorganic filler(s) 5. other additives (biocide, antifreeze)
Binders Binders are often supplied as an aqueous latex emulsion (40e60% solids of polymeric resin binder, in water). Polyvinyl acetate (PVAc) polymers and copolymers are the well-known polymeric resins suitable for binders for wall repair compounds. Other resins such as ethylene vinyl acetate polymers, styrene-butadiene polymers, polyacrylamide polymers, natural and synthetic starch, natural rubber latex, and casein can also be used as binders alone or in combination with other binders. Ideally, the polymeric resin binder should have a glass transition temperature (Tg) of around room temperature (from about 20 C to about 30 C). A Tg in this temperature range renders the binder well suited for fusing and coalescing
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under ambient conditions after the wall repair compound has been applied and allowed to dry. If the Tg is excessively higher than RT, the binder might be below its Tg in the dried compound and render the compound relatively brittle and prone to cracking. Conversely, too low a glass transition might render the wall repair compound too soft or rubbery for sanding. It is also beneficial if the binder has a relatively broad Tg range (w10 C) such that the binder does not exhibit a relatively sharp change in physical properties upon changes in ambient temperature.
Thickeners and Cothickeners Conventional wall repair compounds often comprise organic polymeric thickeners. Such organic polymeric thickeners are often used to provide an increased viscosity of the wall repair compound so that the compound does not excessively sag, slump, or run when applied to a vertical wall. They are often designed to exhibit their thickening effect by their interaction with the water that is present in the wall repair compound. Therefore, they are often water soluble or water swellable (around 22 C) due to the presence of multiple hydroxyl groups in their structure. They are commonly referred to as water retention agents in the industry. The most well-known polyhydroxy compounds are cellulose ethers (e.g., methyl cellulose, hydroxypropyl cellulose, hydroxypropyl methyl cellulose, hydroxyethyl cellulose, hydroxyethyl methyl cellulose, hydroxyethyl hydroxypropyl cellulose, ethylhydroxyethyl cellulose, and sodium carboxymethyl cellulose). Such thickeners can also include, for example, polyethylene glycol, polyethylene oxide (and/or polyethylene oxide/ polypropylene oxide copolymers), PVA polymers or copolymers.
Synthetic Inorganic Fillers Certain natural or synthetic inorganic fillers (e.g., clays such as attapulgite, bentonite, montmorillonite, illite, kaolinite, sepiolite), while not necessarily water soluble, are known to exhibit a thickening (e.g., viscosity-increasing) effect when dispersed in water. Such materials (particularly those that absorb water and/or swell upon exposure to water) have commonly been used as thickeners in wall repair compounds (they are also occasionally referred to as rheology modifiers, nonleveling agents, slip agents, etc.), and are known to contribute to shrinkage upon drying as discussed in US 4,824,879 [4]. These clay fillers also work as a cothickener with cellulosic, associative, and alkali swellable thickeners.
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Water Wall repair compounds often comprise a significant amount of water (e.g., greater than about 20% by weight), such that, after the wall repair compound is applied to a wall, the water evaporates over a period of time resulting in the formation of a dried, hardened material which can be sanded, painted, etc.
Inorganic Filler(s) Inorganic fillers used in wall repair compounds include talc, CaCO3, and low density fillers such as HGMs. Because of the use of HGMs, the spackling are lightweight and provide less trowel drag with great nonleveling characteristics. One other advantage to using HGMs is that there is minimal shrinkage. At 66% by volume, the spheres are packed in as tightly as possible. Therefore, even when the water is removed, the system as a whole does not move. Talc cannot be packed as tightly so when the water is removed, there is movement between the talc particles resulting in shrinkage. Another benefit is reduced drying time. With the lower water level, the drying time is reduced and the amount of labor is reduced. The use of HGMs in a spackling composition was shown earlier in references US 3,386,223 [5] in 1968 and US 4,391,647 [6] in 1983. A spackle composition containing HGMs from US 4,391,647 [6] is shown in Table 9.5. The composition was reported to be also suitable for use as a stucco material, for example, in refurbishing ceilings and it provided fire resistance and insulating values. Borate-containing HGMs were reported to cause quick gelling in the presence of PVA binder reacting with the hydroxyl groups [7]. Although the problem was more pronounced when PVA was used as a binder alone, it was also present to a lesser degree when PVA was added in certain amount in the PVAc emulsion. Guar gum, a naturally occurring high molecular weight polyhydroxy compound, similarly gels in the presence of borate. Gelation may result in other cases where high molecular weight polyhydroxy compounds are present in the wall repair compound used for water retention, viscosity control, latex stabilization, etc. Although gelation may be counteracted by acidifying the system, other problems could result. For example, fillers such as calcium carbonate can dissolve in acidic systems, releasing carbon dioxide and thus causing intolerable bubbling. US 4,629,751 [7] selectively employed a low molecular weight polyhydroxy compound in a borate-containing environment to prevent an undesirable reaction with a high molecular weight polyhydroxy compound. The composition shown in Table 9.6 was reported to be free of
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Table 9.5 Spackle composition containing HGMs US 4,391,647 [6] %
%
%
Water
47.9700
47.2
50.0
Tamol SN surfactant
0.8722
0.9
0.99
Triton X-100 surfactant
0.4361
0.5
0.5
Attapulgus clay (cothickener)
3.4887
5.0
Methocel (water retention agent)
0.7414
0.8
0.8
Dowicil 75 (anti microbial) 1-(3-chloroallyl)3,5,7-triaza1-azoniaadamantane chloride
0.0044
0.01
0.01
Talc
7.8496
8.0
8.8
Mica
2.4421
2.6
2.7
Marble white
27.9098
29.99
31.0
Polyvinyl acetate
4.3609
0.8
0.8
HGMs (0.15 g/cc eD50 50 mm)
3.9248
4.2
4.4
Reported benefits
Excellent adhesion to the spackling knife, little or no trowel drag, good workability, lightweight, nonleveling properties, and good mixability with normal drying time and tape adhesion.
Exhibited excellent workability, slip, resistance to flow, good tape bonding, and minimum cracking
Same as second but with better adhesion to holes and crevices
HGM, hollow glass microsphere.
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Table 9.6 Spackle composition utilizing low molecular weight polyhydroxy compound (sorbitol) US 4,629,751 (1986) [7] Parts by Weight
Calculated Vol %, Solid Basis
5% (weight) aqueous solution of PVA (binder)
800
3.68
Ethylene glycol
15
1.38
Preservative
5.5
0.51
Attapulgus clay
18
0.64
Ground calcium carbonate
244
8.33
Talc
77
2.53
Mica powder
36
1.45
Thixotrope (hydroxyethyl cellulose)
5
0.46
Sorbitol (low molecular weight polyhydroxy compound)
12
1.11
HGM (0.15 g/cc, 250 psi)
130
79.91
HGM, hollow glass microsphere.
gelation, even after standing for several days. When applied to gypsum board, the composition was reported to adhere well to seam tape, showing little or no signs of sagging or leveling when applied to vertical surfaces. Recently, the use of smoothing agents, such as glycol ethers, was taught in US 8,653,158 when preparing a wall repair compound with low shrinkage [8]. Such smoothing agents appear to function by reducing the apparent viscosity of the wall repair compound rather than increasing it, while not causing unacceptable sagging or slumping. The presence of such a smoothing agent was also reported to improve the ability of the compound to be brought back to the above-described smooth consistency by the addition of a small amount of water, in the event that the compound is inadvertently allowed to lose water (e.g., by the container being left open for a period of time). In the absence of such a smoothing agent, the addition of water only served to reduce the viscosity of the compound such that unacceptable sagging or slumping resulted. In the reference, the smoothing agents employed in the wall repair compound comprised a bimodal mixture of substantially spherical synthetic inorganic fillers such as HGMs and ceramic microspheres.
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Table 9.7 Low shrink wall repair compound employing low molecular weight glycol ether smoothing agent Lightweight Wall Repair Compounds. US 8,653,158 (2014) [8] Component
Wt%
UCAR binder emulsion 626
57.32
K20-3M HGM
25.95
Ceramic microspheres
15.94
Propylene glycol Butyl ether
0.24
Polyphase P20T
0.42
Mergal 192
0.11
HGM, hollow glass microsphere.
Glycol ethers are low molecular weight (e.g., from about 90 g/mole to about 250 g/mole) and are typically liquid at room temperature (e.g., 220 C). While being partially or completely miscible with water, they do not substantially increase the viscosity when added to the water phase Table 9.7.
Tape Joint Compound During installation, gypsum board surfaces frequently develop cracks, pits, etc. When gypsum board is mounted, there are inevitably dimples at the location where individual sheets are nailed or screwed to studs or ceiling joists. A strip of perforated tape is commonly applied over the indented adjacent edges of a space between adjoining gypsum board panels, spreadable joint filling compound being applied both under and over the tape and allowed to dry or cure. Similar to spackling compounds, tape joint compounds include polymeric binder, filler, and water, but also typically include preservatives, water retention agents, wetting agents, defoamers, plasticizers, nonleveling agents, etc. US 4,824,879 [4] describes a low-shrinkage tape joint compound. Tape joint compounds typically include enough water to permit them to be readily and smoothly applied with a spatula or trowel. A substantial amount of shrinkage typically results during drying of the water. It is thus generally necessary to apply such compounds in several separate thin coats, sanding at least after the last coat has dried, in order to avoid leaving a disfiguring depression where the joint has been filled. For example,
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when filling taped joints between abutted gypsum board panels, it is usually necessary to apply three coats of a joint filling compound having a shrinkage of 30e40%, drying and sanding between applications. If tape joint compound shrinkage is 20e30%, it may require only two applications. Tabulated in Table 9.8 is a tape joint compound with low shrinkage, 14%. The low shrinkage was achieved by minimizing the volume percentage of water adsorbing additives such as the conventionally included attapulgite and hydroxyethyl cellulose, thereby obtaining compounds having lower water content. This decreased shrinkage was reported to significantly reduce the need for additional coatings, even where the area to be filled is deep or extensive, and thereby greatly lowers labor cost. In preparing these formulations the attapulgite and “Tamol” dispersants were vigorously mixed with approximately half of the water, using an air motor and a high shear mixer so as to thoroughly disperse the clay. This dispersion was combined with the PVAc (supplied as an aqueous emulsion) and preservative, using a Ross double planetary mixer. Table 9.8 Low-shrinkage tape joint compound Component
Wt%
Vol%
Polyvinyl acetate (40% aq emulsion)
1.9
3.0
HGM
0.51
6.0
Attapulgite
0.68
0.5
Hydroxyethyl cellulose
0.12
0.3
Calcium carbonate
70.0
45.4
Nuosept 95 preservative
0.01
0.0003
Tamol 850 dispersant
0.34
0.5
Mica
1.6
1.0
Water
24.8
43.3
Talc
Density, g/cc Viscosity, Brookfield RVT, spindle F Shrinkage % HGM, hollow glass microsphere.
1.8 2800 14
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The remaining dry ingredients were blended by hand and added to the planetary mixer. The ingredients were then mixed for 5 min, adding just enough water to provide a stiff but fluid mass. After the sides of the mixing vessel had been scraped down, the remaining additional water was added to obtain the desired viscosity, on the order of 2000e3000 Pa.s.
Final Word Repair compounds, whether it is auto, wall, tape, ceiling, or stucco, benefit greatly from the use of hollow glass microspheres due to their low density, excellent spherical packing leading to high solids content with no sagging and low shrinkage, great nonleveling characteristics, and less trowel drag.
References [1] Pechacek et al., Composition for Filling, Patching and the Like. US 3,873,475, March 25, 1975. [2] A.W. Holle, Polyester Based Patching Composition. US 4,053,448, October 11, 1977. [3] P.E. Naton, Plastic Body Filler. US 4,980,414, December 25, 1990. [4] Montgomery et al., Low Shrinkage Tape Joint Composition Containing Attapulgite. US 4,824,879, April 25 1989. [5] A.A. Wegwerth, Method of Joining Drywall Panels. US 3,386,223, June 4, 1968. [6] Deer et al., Spackeling Composition. US 4,391,647, July 5, 1983. [7] R.L. Montgomery, Gel-resistant Bubble-filled Wall Repair Compound. US 4,629,751, December 16, 1986. [8] Gozum et al., Lightweight Wall Repair Compounds. US 8,653,158, Feburary 18, 2014.
Hollow glass microspheres (HGMs) have been a key component of repair compounds for auto, wall, plaster, etc., since the 1960s. In this chapter, we will briefly discuss the use and benefits of HGMs in these application areas.
Auto Repair Compounds An auto repair compound is used to repair and patch damaged or dented sheet metals. Metal surfaces of automobile bodies (and household appliances) having dents, cracks, or holes are typically repaired with inorganic filler containing thermosetting unsaturated polyester patching compound in order to eliminate these imperfections (Figure 9.1). A satisfactory elimination of such imperfections involves subjecting the dents filled with the hardened patching material to a finishing operation in order to obtain a smooth, uniform surface which blends with the metal surface being repaired so as to be indistinguishable from the rest of the metal surface after a coat of paint has been applied to the surface. Since this subsequent finishing operation is costly and time consuming, minimizing the time required for the finishing operation is of great importance and HGMs play
Figure 9.1 Auto repair compound being applied to a sheet metal of automobile bodies. Hollow Glass Microspheres for Plastics, Elastomers, and Adhesives Compounds http://dx.doi.org/10.1016/B978-1-4557-7443-2.00009-8 Copyright © 2015 Elsevier Inc. All rights reserved.
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an important role in reducing the time required for finishing operation. In addition, HGMs render the patching material readily workable, that is, ability to file and sand, with good featherability, that is, ability to blend in with adjacent metal surfaces. Unsaturated polyesters are the typical binder ingredient of an auto repair compound. The unsaturated polyesters used in the patching compounds are usually dissolved in styrene monomer, usually 10e30 part styrene to 90e70 parts by weight of the unsaturated polyester. We have discussed the use of unsaturated polyesters in fiber reinforced sheet molding compound (SMC) applications in a prior chapter. As described in Chapter X, the unsaturated polyester itself is usually formed by the esterification of glycols with dicarboxylic acids or acid anhydrides. One obvious difference in a repair compound versus structural articles made by SMC (or the like) is the absence of glass fibers in a repair compound formula. Typical ingredients of an auto repair compound comprises unsaturated polyester resin fillers (talc, HGMs, and sometimes calcium carbonate) thixotrope and flow agent (e.g., cellulose acetate butyrate (CAB)) hardenerdadded just before using The mixture of thermosetting polyester resin binder/flow additives and inorganic fillers has the consistency of a paste. The filled polyester patching paste and the hardener (cross-linking catalyst such as benzoyl peroxide, cumyl peroxide, and methyl ethyl ketone peroxide) components are packaged in separate containers and blended together (0.5e5 wt%) immediately prior to application on a dented surface to accelerate hardening of the patching compound. At the time of application, the filled thermosetting polyester patching compound is partially gelled and thixotropic and can be formed and shaped to the configuration of the dent, and upon cure, the patching compound becomes a hardened thermoset mass of resin which is confined within the dent. Table 9.1 shows the ingredients of a metal patching compound (0.77 g/cc and 220,000 cps peak viscosity) where the fillers were incorporated in the polyester resin in major amounts, for example, the total amount of filler particles incorporated in the polyester resin constitutes one-half to twothirds the total volume of the patching composition [1]. This compound was prepared by first mixing uPS and cellulose acetate butyrate (CAB) in a low speed spiral mixer at 80 C until a clear solution is obtained and cooled to 20 C. TiO2 and the fillers (talc and HGMs) were added subsequently.
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Table 9.1 Metal patching compound (0.77 g/cc and 220,000 cps peak viscosity) US Pat. No. 3,873,475 [1] Parts Polyester:styrene solution (70:30)w/accelerator dimethyl aniline
58.7
Cellulose acetate butyrate (CAB)
1.8
TiO2 rutile
1.0
Talc (10 mm)
23.0
HGM (0.2 g/cc)
15.5
HGM, hollow glass microsphere.
After mixing to produce a uniform blend, a vacuum of 635 mm Hg was applied and mixing continued until a smooth, viscous composition, homogeneous in appearance and free of air bubbles formed. The final compound density was 0.77 g/cc and it had a viscosity of 220,000 cps. In another early reference [2] (US 4,053,448), it was shown that the proper selection of high density fillers (silica to talc ratio) was a critical feature of the formulation to provide superior workability properties. Table 9.2 shows the ingredients of the auto body repair compound discussed in this reference. The effect of varying amounts of amorphous silica/talc filler admixture on the performance of auto repair compound is shown in Table 9.3. It shows that the silica/talc ratio of 1:1 to 1:2 gives the best results as far as adhesion, fileability, and sandability. Adhesion was a measure of firm bonding of a sufficiently hardened compound to the metal surface 7 min after catalyst Table 9.2 Auto repair compound comprising silica/talc high density filler admixture and HGMs (US 4,053,448) [2] Parts Polyester:styrene solution (70:30)
42
TiO2 rutile
1.0
HGM (0.23 g/cc, 500 psi) low density filler
7
Silica/Talc high density filler admixture
50
Benzoyl peroxide
4
HGM, hollow glass microsphere.
218
Table 9.3 Performance of auto repair compound with varying silica/talc filler admixture Filler Admixture
Gel
Elapsed
Silica Talc Time Adhesion
Elapsed
Fileability
Time
Sandability
Time
Paintability
Min
Rating
Min
Rating
%
Min
Rating
Rating
1
33
17
2.5
Good
Good
7
Good
12
Excellent
2
25
25
2.5
Excellent
Excellent
7
Excellent
12
Excellent
3
17
33
4.5
Excellent
Excellent
10
Excellent
14
Good
4
8
42
5.0
Good
Good
12
Good
20
Poor
5
42
8
2.5
Poor
Poor
7
Poor
12
Excellent
6
50
0
2.5
Poor
Poor
14
Poor
20
Excellent
8
0
50
5.5
Poor
Poor
14
Poor
20
Poor
9
46
4
2.5
Poor
Poor
7
Poor
12
Excellent
10
4
46
5.5
Poor
Poor
14
Poor
20
Poor
H OLLOW G LASS M ICROSPHERES
%
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Table 9.4 Ingredients of an auto patching compound useful for repairing galvanized steel Ingredients
Wt%
uPS resin
33
Styrene monomer
15
Toluene sulfonamideeformaldehyde resin
3
Talc
44
HGM
5
HGM, hollow glass microsphere.
addition. Fileability was a measure of the ease of hand filing the hardened patching composition, that is, if the hardened patching composition was soft and pliable enough to be hand filed, after 7e14 min. Sandability was a measure of whether or not the patching composition applied to the dent can be sanded to trade specifications after 12e20 min have elapsed from the time of catalyst addition to the patching composition. Paintability was rated based on appearance being free of pinholes and blisters. In reference US 4,980,414 [3], it was shown that a uPS-based patching compound that includes aryl sulfonamide-aldehyde resin could be used for repairing metal surfaces with galvanized steel (Table 9.4). When the metal requiring repair is galvanized steel, it is difficult to ensure adhesion of the body filler to the galvanized steel surfaces. Generally, the galvanized steel surfaces are pretreated in order to provide the desirable adhesion of the body filler to the galvanized steel. The pretreatment generally involves thoroughly grinding the galvanized surface to remove the zinc coating, sanding of the surface, acid etching of the surface to remove any remaining galvanized coating, neutralization of the acid, followed by cleaning and drying of the surface. In many instances, this treated surface is coated with an epoxy/urethane primer to ensure corrosion protection (since the galvanized surface has been removed) followed by light sanding of the primer. The body filler can then be applied to the pretreated galvanized steel surfaces, and the adhesion of the body filler to the surface is generally acceptable. However, this process, which is required to prepare galvanized steel for body fillers is both time consuming and costly. Also, the corrosion protection is lost by removal of the galvanized surface provided by the primer which is often not the equivalent of the corrosion protection afforded by the original galvanized surface.
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Figure 9.2 Heavyweight versus lightweight spackle in plasterboard.
Wall Repair (Spackle Compounds) Interior walls of buildings are often constructed using gypsum wallboard panels (sometimes referred to as drywall, plasterboard). Where cavities, recesses, holes, etc., may be present (due to imperfections or damage) it is common to use wall repair compound (often referred to as “spackling”) to fill such cavities (Figure 9.2). Conventional wall repair compounds often include 1. polymeric resin binder(s) (e.g., polyvinyl alcohol (PVA) emulsion in water) 2. thickeners 3. water 4. inorganic filler(s) 5. other additives (biocide, antifreeze)
Binders Binders are often supplied as an aqueous latex emulsion (40e60% solids of polymeric resin binder, in water). Polyvinyl acetate (PVAc) polymers and copolymers are the well-known polymeric resins suitable for binders for wall repair compounds. Other resins such as ethylene vinyl acetate polymers, styrene-butadiene polymers, polyacrylamide polymers, natural and synthetic starch, natural rubber latex, and casein can also be used as binders alone or in combination with other binders. Ideally, the polymeric resin binder should have a glass transition temperature (Tg) of around room temperature (from about 20 C to about 30 C). A Tg in this temperature range renders the binder well suited for fusing and coalescing
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under ambient conditions after the wall repair compound has been applied and allowed to dry. If the Tg is excessively higher than RT, the binder might be below its Tg in the dried compound and render the compound relatively brittle and prone to cracking. Conversely, too low a glass transition might render the wall repair compound too soft or rubbery for sanding. It is also beneficial if the binder has a relatively broad Tg range (w10 C) such that the binder does not exhibit a relatively sharp change in physical properties upon changes in ambient temperature.
Thickeners and Cothickeners Conventional wall repair compounds often comprise organic polymeric thickeners. Such organic polymeric thickeners are often used to provide an increased viscosity of the wall repair compound so that the compound does not excessively sag, slump, or run when applied to a vertical wall. They are often designed to exhibit their thickening effect by their interaction with the water that is present in the wall repair compound. Therefore, they are often water soluble or water swellable (around 22 C) due to the presence of multiple hydroxyl groups in their structure. They are commonly referred to as water retention agents in the industry. The most well-known polyhydroxy compounds are cellulose ethers (e.g., methyl cellulose, hydroxypropyl cellulose, hydroxypropyl methyl cellulose, hydroxyethyl cellulose, hydroxyethyl methyl cellulose, hydroxyethyl hydroxypropyl cellulose, ethylhydroxyethyl cellulose, and sodium carboxymethyl cellulose). Such thickeners can also include, for example, polyethylene glycol, polyethylene oxide (and/or polyethylene oxide/ polypropylene oxide copolymers), PVA polymers or copolymers.
Synthetic Inorganic Fillers Certain natural or synthetic inorganic fillers (e.g., clays such as attapulgite, bentonite, montmorillonite, illite, kaolinite, sepiolite), while not necessarily water soluble, are known to exhibit a thickening (e.g., viscosity-increasing) effect when dispersed in water. Such materials (particularly those that absorb water and/or swell upon exposure to water) have commonly been used as thickeners in wall repair compounds (they are also occasionally referred to as rheology modifiers, nonleveling agents, slip agents, etc.), and are known to contribute to shrinkage upon drying as discussed in US 4,824,879 [4]. These clay fillers also work as a cothickener with cellulosic, associative, and alkali swellable thickeners.
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Water Wall repair compounds often comprise a significant amount of water (e.g., greater than about 20% by weight), such that, after the wall repair compound is applied to a wall, the water evaporates over a period of time resulting in the formation of a dried, hardened material which can be sanded, painted, etc.
Inorganic Filler(s) Inorganic fillers used in wall repair compounds include talc, CaCO3, and low density fillers such as HGMs. Because of the use of HGMs, the spackling are lightweight and provide less trowel drag with great nonleveling characteristics. One other advantage to using HGMs is that there is minimal shrinkage. At 66% by volume, the spheres are packed in as tightly as possible. Therefore, even when the water is removed, the system as a whole does not move. Talc cannot be packed as tightly so when the water is removed, there is movement between the talc particles resulting in shrinkage. Another benefit is reduced drying time. With the lower water level, the drying time is reduced and the amount of labor is reduced. The use of HGMs in a spackling composition was shown earlier in references US 3,386,223 [5] in 1968 and US 4,391,647 [6] in 1983. A spackle composition containing HGMs from US 4,391,647 [6] is shown in Table 9.5. The composition was reported to be also suitable for use as a stucco material, for example, in refurbishing ceilings and it provided fire resistance and insulating values. Borate-containing HGMs were reported to cause quick gelling in the presence of PVA binder reacting with the hydroxyl groups [7]. Although the problem was more pronounced when PVA was used as a binder alone, it was also present to a lesser degree when PVA was added in certain amount in the PVAc emulsion. Guar gum, a naturally occurring high molecular weight polyhydroxy compound, similarly gels in the presence of borate. Gelation may result in other cases where high molecular weight polyhydroxy compounds are present in the wall repair compound used for water retention, viscosity control, latex stabilization, etc. Although gelation may be counteracted by acidifying the system, other problems could result. For example, fillers such as calcium carbonate can dissolve in acidic systems, releasing carbon dioxide and thus causing intolerable bubbling. US 4,629,751 [7] selectively employed a low molecular weight polyhydroxy compound in a borate-containing environment to prevent an undesirable reaction with a high molecular weight polyhydroxy compound. The composition shown in Table 9.6 was reported to be free of
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Table 9.5 Spackle composition containing HGMs US 4,391,647 [6] %
%
%
Water
47.9700
47.2
50.0
Tamol SN surfactant
0.8722
0.9
0.99
Triton X-100 surfactant
0.4361
0.5
0.5
Attapulgus clay (cothickener)
3.4887
5.0
Methocel (water retention agent)
0.7414
0.8
0.8
Dowicil 75 (anti microbial) 1-(3-chloroallyl)3,5,7-triaza1-azoniaadamantane chloride
0.0044
0.01
0.01
Talc
7.8496
8.0
8.8
Mica
2.4421
2.6
2.7
Marble white
27.9098
29.99
31.0
Polyvinyl acetate
4.3609
0.8
0.8
HGMs (0.15 g/cc eD50 50 mm)
3.9248
4.2
4.4
Reported benefits
Excellent adhesion to the spackling knife, little or no trowel drag, good workability, lightweight, nonleveling properties, and good mixability with normal drying time and tape adhesion.
Exhibited excellent workability, slip, resistance to flow, good tape bonding, and minimum cracking
Same as second but with better adhesion to holes and crevices
HGM, hollow glass microsphere.
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Table 9.6 Spackle composition utilizing low molecular weight polyhydroxy compound (sorbitol) US 4,629,751 (1986) [7] Parts by Weight
Calculated Vol %, Solid Basis
5% (weight) aqueous solution of PVA (binder)
800
3.68
Ethylene glycol
15
1.38
Preservative
5.5
0.51
Attapulgus clay
18
0.64
Ground calcium carbonate
244
8.33
Talc
77
2.53
Mica powder
36
1.45
Thixotrope (hydroxyethyl cellulose)
5
0.46
Sorbitol (low molecular weight polyhydroxy compound)
12
1.11
HGM (0.15 g/cc, 250 psi)
130
79.91
HGM, hollow glass microsphere.
gelation, even after standing for several days. When applied to gypsum board, the composition was reported to adhere well to seam tape, showing little or no signs of sagging or leveling when applied to vertical surfaces. Recently, the use of smoothing agents, such as glycol ethers, was taught in US 8,653,158 when preparing a wall repair compound with low shrinkage [8]. Such smoothing agents appear to function by reducing the apparent viscosity of the wall repair compound rather than increasing it, while not causing unacceptable sagging or slumping. The presence of such a smoothing agent was also reported to improve the ability of the compound to be brought back to the above-described smooth consistency by the addition of a small amount of water, in the event that the compound is inadvertently allowed to lose water (e.g., by the container being left open for a period of time). In the absence of such a smoothing agent, the addition of water only served to reduce the viscosity of the compound such that unacceptable sagging or slumping resulted. In the reference, the smoothing agents employed in the wall repair compound comprised a bimodal mixture of substantially spherical synthetic inorganic fillers such as HGMs and ceramic microspheres.
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Table 9.7 Low shrink wall repair compound employing low molecular weight glycol ether smoothing agent Lightweight Wall Repair Compounds. US 8,653,158 (2014) [8] Component
Wt%
UCAR binder emulsion 626
57.32
K20-3M HGM
25.95
Ceramic microspheres
15.94
Propylene glycol Butyl ether
0.24
Polyphase P20T
0.42
Mergal 192
0.11
HGM, hollow glass microsphere.
Glycol ethers are low molecular weight (e.g., from about 90 g/mole to about 250 g/mole) and are typically liquid at room temperature (e.g., 220 C). While being partially or completely miscible with water, they do not substantially increase the viscosity when added to the water phase Table 9.7.
Tape Joint Compound During installation, gypsum board surfaces frequently develop cracks, pits, etc. When gypsum board is mounted, there are inevitably dimples at the location where individual sheets are nailed or screwed to studs or ceiling joists. A strip of perforated tape is commonly applied over the indented adjacent edges of a space between adjoining gypsum board panels, spreadable joint filling compound being applied both under and over the tape and allowed to dry or cure. Similar to spackling compounds, tape joint compounds include polymeric binder, filler, and water, but also typically include preservatives, water retention agents, wetting agents, defoamers, plasticizers, nonleveling agents, etc. US 4,824,879 [4] describes a low-shrinkage tape joint compound. Tape joint compounds typically include enough water to permit them to be readily and smoothly applied with a spatula or trowel. A substantial amount of shrinkage typically results during drying of the water. It is thus generally necessary to apply such compounds in several separate thin coats, sanding at least after the last coat has dried, in order to avoid leaving a disfiguring depression where the joint has been filled. For example,
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when filling taped joints between abutted gypsum board panels, it is usually necessary to apply three coats of a joint filling compound having a shrinkage of 30e40%, drying and sanding between applications. If tape joint compound shrinkage is 20e30%, it may require only two applications. Tabulated in Table 9.8 is a tape joint compound with low shrinkage, 14%. The low shrinkage was achieved by minimizing the volume percentage of water adsorbing additives such as the conventionally included attapulgite and hydroxyethyl cellulose, thereby obtaining compounds having lower water content. This decreased shrinkage was reported to significantly reduce the need for additional coatings, even where the area to be filled is deep or extensive, and thereby greatly lowers labor cost. In preparing these formulations the attapulgite and “Tamol” dispersants were vigorously mixed with approximately half of the water, using an air motor and a high shear mixer so as to thoroughly disperse the clay. This dispersion was combined with the PVAc (supplied as an aqueous emulsion) and preservative, using a Ross double planetary mixer. Table 9.8 Low-shrinkage tape joint compound Component
Wt%
Vol%
Polyvinyl acetate (40% aq emulsion)
1.9
3.0
HGM
0.51
6.0
Attapulgite
0.68
0.5
Hydroxyethyl cellulose
0.12
0.3
Calcium carbonate
70.0
45.4
Nuosept 95 preservative
0.01
0.0003
Tamol 850 dispersant
0.34
0.5
Mica
1.6
1.0
Water
24.8
43.3
Talc
Density, g/cc Viscosity, Brookfield RVT, spindle F Shrinkage % HGM, hollow glass microsphere.
1.8 2800 14
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The remaining dry ingredients were blended by hand and added to the planetary mixer. The ingredients were then mixed for 5 min, adding just enough water to provide a stiff but fluid mass. After the sides of the mixing vessel had been scraped down, the remaining additional water was added to obtain the desired viscosity, on the order of 2000e3000 Pa.s.
Final Word Repair compounds, whether it is auto, wall, tape, ceiling, or stucco, benefit greatly from the use of hollow glass microspheres due to their low density, excellent spherical packing leading to high solids content with no sagging and low shrinkage, great nonleveling characteristics, and less trowel drag.
References [1] Pechacek et al., Composition for Filling, Patching and the Like. US 3,873,475, March 25, 1975. [2] A.W. Holle, Polyester Based Patching Composition. US 4,053,448, October 11, 1977. [3] P.E. Naton, Plastic Body Filler. US 4,980,414, December 25, 1990. [4] Montgomery et al., Low Shrinkage Tape Joint Composition Containing Attapulgite. US 4,824,879, April 25 1989. [5] A.A. Wegwerth, Method of Joining Drywall Panels. US 3,386,223, June 4, 1968. [6] Deer et al., Spackeling Composition. US 4,391,647, July 5, 1983. [7] R.L. Montgomery, Gel-resistant Bubble-filled Wall Repair Compound. US 4,629,751, December 16, 1986. [8] Gozum et al., Lightweight Wall Repair Compounds. US 8,653,158, Feburary 18, 2014.